Population Interactions
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Direct answer
Two species sharing a habitat interact in ways classified by their effect on each partner: predation (+/−), competition (−/−), parasitism (+/−), commensalism (+/0) and mutualism (+/+), with amensalism (−/0) completing the scheme. These interactions regulate population sizes, shape community structure and drive coevolution, and NCERT treats them under organisms and populations with a fixed set of examples that the examination repeatedly recycles.
What you must remember
- Predation: the conductor of energy flow from plants to herbivores and up the trophic ladder; it keeps prey populations in check — predators like the moth Cactoblastis cactorum famously controlled the runaway prickly pear cactus in Australia, a textbook example of biological control.
- Prey defences: camouflage and unpalatability, the monarch butterfly sequestering chemicals from its larval host plant being the standard example.
- Competition: Gause's competitive exclusion principle — two species with overlapping needs for the same limiting resource cannot coexist indefinitely; the Abingdon tortoise of the Galapagos went extinct after goats out-grazed it.
- Parasitism: ectoparasites such as lice on humans and ticks on dogs live on the host, endoparasites such as Plasmodium and Ascaris live within; parasitic plants like Cuscuta drain host sap.
- Brood parasitism: the cuckoo lays its eggs in the crow's nest and the host incubates them — a behavioural parasitism that examiners love.
- Commensalism: the orchid growing as an epiphyte on a mango branch, barnacles on a whale, and the cattle egret foraging beside grazing cattle all benefit one party without harming the other.
- Mutualism: lichens (alga and fungus), mycorrhizal roots, and the species-specific fig-wasp relationship in which the wasp pollinates the fig and breeds inside its fruit.
Common confusion
Commensalism and mutualism are the habitual mix-up: the egret-and-cattle pairing is commensalism because only the bird profits, whereas fig and wasp are mutualism because both species are obligate partners. Parasitism differs from predation in that the parasite feeds on a living host over time rather than killing and consuming it quickly, and brood parasitism exploits parental care rather than body tissue.
Exam-focused takeaway
NEET-UG builds match-the-following questions almost entirely from the NCERT example list — cuckoo with brood parasitism, orchid with commensalism, fig-wasp with mutualism, Abingdon tortoise with competitive exclusion. Assertion-reason stems test Gause's principle and why predators are necessary for community stability, and statement-based questions classify a described interaction as mutualism, commensalism or parasitism. Learn the sign notation (+/−, +/0, +/+) alongside each example.
Frequently asked questions
What is Gause's competitive exclusion principle?
It states that two closely related species competing for the same resources cannot coexist indefinitely, and the competitively inferior one is eliminated — as happened to the Abingdon tortoise after goats were introduced.
What is brood parasitism?
A bird, classically the cuckoo, laying its eggs in another species' nest so the host, such as a crow, incubates and raises the parasite's young.
How does commensalism differ from mutualism?
In commensalism only one partner benefits while the other is unaffected, whereas in mutualism both partners benefit — the egret and cattle versus the fig and its wasp.
What are ectoparasites and endoparasites?
Ectoparasites live on the body of the host, like lice and ticks; endoparasites live inside it, like the liver fluke, Plasmodium and Ascaris.
Give standard examples of mutualism.
Lichens, the mycorrhizal association between fungi and plant roots, and the species-specific pollination mutualism between fig trees and wasps.